1. Field of the Invention
The present invention relates generally to recording of digital signals on optical or magnetic discs and more specifically to a data modulation method and apparatus for converting a data bit stream to a channel bit stream having a small d.c. content (or digital sum value).
2. Description of the Related Art
Prior to recording on an optical or magnetic disc, the format of a digital signal is converted, or modulated into a format appropriate for the characteristics of the disc. DC content is a concern in digital signals, which alternate between “1” and “0” irregularly. If there is an imbalance between the total number of ones or zeros, the imbalance will result in a signal having a d.c. content. DC content is a concern with magnetic recording heads. If the magnetic polarity is switched more to one direction than the other, this polarity imbalance will remain in the head and cause a d.c. offset in its ability to play back digital signals. DC content is also a concern with a laser mechanism which reads pits and lands on the surface of an optical disc. In a digital system, the d.c. content is termed digital sum value (DSV), which is the difference between the total number of ones and the total number of zeros. If NRZI waveform is used for recording, the DSV is a total sum of +1 for a bit “1” and −1 for a bit “0”, starting from the beginning of a channel bit stream. The pits and lands are read by a system of lasers that are controlled by a servo actuator. The actuator must maintain precise alignment with the tracks on the disc and with the transition boundaries between pit and land areas. The transitions in the signal, represented by the ones, enable the servo actuator to maintain correct alignment with the track. Thus, a signal having a d.c. content will result in the servo actuator drifting out of proper alignment with the track.
In DVD (Digital Versatile Disc) recording systems, the 8-16 modulation scheme is employed to convert 8-bit data words of a data bit stream to 16-bit code words to form a channel bit stream so that the run length of the channel bit stream is constrained to (2, 10), i.e., the number of 0's that exist between any two 1's is a minimum of two and a maximum of ten. The 8-16 modulation has the ability to reduce the d.c. content of a signal by selecting a particular code word from a set of tables based on the DSV of the signal.
The 8-16 modulation has a coding rate 1/2. The coding rate is of a concern to the design of a recording system because higher the coding rate a longer time is allowed to detect a “1” bit from the channel bit stream. The (1, 7) modulation is another coding scheme known in the art as having a run length of a minimum of one “0” and a maximum of seven 0's and a coding rate 2/3. Because of its higher coding rate, the (1, 7) modulation is suitable for high-density digital recording. However, its d.c. content is high, which results in a degraded jitter performance if an a.c.-coupled circuit is used in a playback system and results in a decision threshold varying randomly about the correct level.
Japanese Patent Publication 1998-340543 discloses a d.c.-content reduction method for the (1, 7) modulation codes by introducing a 6-bit DSV control code of 3 replacement bits and 3 redundant bits into a data bit stream at regular intervals prior to conversion to a channel bit stream. However, the use of the extra 3-bit redundant bits results in a lowering of the coding rate.
Another concern to the design of a recording system is a bit pattern in which bits “1” occur at minimum intervals 2T (where T is the channel bit length) such as “010101010”. The (1, 7) modulation generates such 2T-bit patterns frequently as compared to other modulation schemes. Because of the closely spaced bits “1”, inter-symbol interference occurs, making it difficult to extract clock information from playback signals. Furthermore, noise is introduced to the decision threshold, resulting in a timing error which causes a delayed detection of a full 2T bit pattern by the length of a single bit.